Many thermal mass flow controllers utilize force-actuated “globe” valves to control gas flow through the controllers. A globe valve, as shown in FIG. 1, typically comprises a spherical poppet 10 that seals against a circular or conical valve seat 12. A variable force actuator connected to a valve stem 14 can exert a force to unseat poppet 10 from valve seat 12. A valve, such as that depicted in FIG. 1, can be designed such that the differential pressure force 16 of the fluid being controlled either acts to seal the valve or unseat the valve. In the example of FIG. 1, the differential pressure force 16 acts to unseat the valve. To counteract this force, a preload spring can exert a spring force 18 on the poppet to keep the valve closed in the absence of an actuating force. This preload force is generally set high enough to prevent flow at the highest anticipated inlet pressure.
In many mass flow controllers, the initial application of force by the actuator does not result in appreciable flow, as the valve is held closed by the preload spring force. When the actuating force is approximately equal to the spring force minus the force caused by the differential pressure, the poppet opens and fluid begins to flow. As further actuating force is applied to the valve stem, the poppet is further removed from the valve seat and the flow orifice is enlarged. This causes increased flow through the valve.
FIG. 2 illustrates an example flow/force curve for a valve. The region of the force flow/force curve where no flow occurs is referred to as a deadband. The width of the deadband region is proportional to the preload closing force and the opening force exerted on the poppet by the differential pressure force. The region where flow does occur is the useful control region. The point at which leakage begins to occur and the valve begins to open is referred to as the valve threshold. Flow in the control region is typically proportional to the valve displacement caused by the actuating force acting against the preload spring, valve seat/poppet diameter, and differential pressure. While control region of FIG. 2 is shown as generally linear, the control region can be non linear and suffer from mechanical hysteresis.
Several factors (both dynamic and static) can alter the threshold and gain (i.e., slope of flow/force curve the control region) of the valve. For instance, a change in preload will alter the valve threshold without affecting gain in the control region. A change in valve diameter, on the other hand, will affect the gain with relatively little change in the threshold. This is because the same linear displacement of a larger valve poppet results in greater flow through the valve at the same differential pressure. Similarly, a change in differential pressure will affect gain, with increased differential pressure leading to a larger gain.
FIG. 3 shows three example flow/force curves for three combinations of valve diameters and preload settings. Each of the three response curves is able to reach a one hundred percent flow rate, despite the variance in valve diameter and corresponding preload setting. However, the valves display different threshold levels and gains within their control regions. When the valve diameter is larger or differential pressure anticipated is higher, the valve preload is increased to prevent leak-through. Likewise, when the valve diameter is smaller or the anticipated differential pressure is lower, the preload can be decreased. This compensates for the lower gain of the smaller valve or lower pressure to ensure that the valve can reach an adequate maximum flow rate. In prior art systems, each mass flow controller is individually tuned to compensate for changes in threshold and gain.
Prior art mass flow controller valve systems utilized a single control system output channel to set the opening force applied to the valve stem. When a non-zero mass flow set point is received, the control system begins to increase the output value until flow is established and eventually equals the set point value. The time required to increase the valve force beyond the threshold contributes to a slow response time. Additionally, having to account for the transition through the deadband with a single channel can cause the integral term of the typical control algorithm to overshoot the set point.
To counter these effects, some prior art systems apply a pre-charge to the control system output such that valve drive is driven immediately to the threshold of flow any time a non-zero flow set point is asserted. This system is effective in reducing dead time and control system overshoot, but introduces other problems. One such problem is a loss of resolution in the control region. The actuation force applied to a poppet is generally proportional to the number of quantized step asserted by a digital controller via a digital-to-analog conversion system. Applying a fixed initial force to overcome the valve threshold requires that a single-channel system uses some number of the total available steps for the initial force. As an example, if a single channel control system can adjust the valve opening force in 100 of those steps, the initial force may require 45 of those steps. This leaves only 55 steps to control flow in the control region. As the required threshold force increases, the resolution in the control region decreases as more steps are required for the initial force.
Moreover, the addition of an initial force to reach the valve threshold complicates the control system. Ideally, a control system is most easily implemented when the control output is related to the controlled parameter by a linear function with minimum offset. The use of threshold force, however, introduces a significant offset. The value of the offset can vary greatly based on static and dynamic factors such as valve diameter and differential pressure, making it difficult to calibrate mass flow controllers that apply an initial force through the same channel as the control force.